Method for producing power with stored energy

ABSTRACT

An energy storage system has a pressure vessel that is exposed to ambient temperatures and that contains a working fluid which is condensable at ambient temperatures (CWF); a liquid reservoir in communication with one of the vessels and containing a liquid that is unvaporizable in the reservoir and in the vessel; and apparatus for delivering the liquid from the reservoir to the vessel. The CWF is compressible within the vessel upon direct contact with the liquid and is storable in a liquid state after being compressed to its saturation pressure. In a method, at least some of the liquid located in the vessel is propelled by the CWF towards a turbine to produce power. In one embodiment, a module has a first vessel having at least four ports, a second vessel at ambient temperatures, and a flow control component operatively connected to a corresponding conduit for selectively controlling fluid flow.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No.17/698,236 filed Mar. 18, 2022 and entitled “AN ENERGY STORAGE SYSTEMAND METHOD”.

FIELD OF THE INVENTION

The present invention relates to the field of mechanical energy storage.More particularly, the invention relates to an energy storage systemfrom which compressed gas is releasable for later use and a methodtherefor.

BACKGROUND OF THE INVENTION

Energy collected from various sustainable energy sources such as wind,solar and wave energy sources is known to be stored in the form ofcompressed gas. During periods of power demand, the compressed gas isdischarged from the storage vessel and electrical power is able to begenerated. Alternatively, the stored compressed gas can be utilized indifferent industrial applications.

Many compressed air energy storage (CAES) systems are known in the priorart. Compressed air has traditionally been stored in large and costlyunderground caverns or in underwater elastic balloons. Although CAESplants have a large power rating and storage capacity, they have somemajor drawbacks. Firstly, air has a significantly great heat ofcompression, suffering from a heat loss of approximately 85% whencompressed. Less compressed air is able to be stored as its temperatureincreases. Secondly, the pressure in the cavern within which thecompressed air is stored becomes slowly reduced as additional air isreleased, thereby negatively influencing the amount of electricity thatis able to be produced by a turbine driven by the released compressedair.

Attempts have been made to reduce the heat of compression by chargingand discharging air isothermally; however, the need of employing heatexchangers to facilitate the isothermal compression unnecessarily addscosts to a storage facility.

In other CAES systems, diabatic (D-CAES), adiabatic (A-CAES), and liquid(LAES) air energy storage means have been employed. The air temperaturefor these prior art systems significantly deviates from ambienttemperature, and therefore these prior art systems also require the useof expensive heat exchangers and rotating equipment, i.e. compressorsand gas turbines.

It is an object of the present invention to provide a compressed gasenergy storage system with increased energy density relative to priorart systems.

It is an additional object of the present invention to provide acompressed gas energy storage system with increased system round tripefficiency relative to prior art systems.

It is an additional object of the present invention to provide acompressed gas energy storage system and method that have reducedcapital and operating costs relative to the prior art.

Other objects and advantages of the invention will become apparent asthe description proceeds.

SUMMARY OF THE INVENTION

A multiphase energy storage system comprises at least one pressurevessel that is exposed to ambient temperatures and that is adapted tocontain a condensable working fluid (CWF) which is condensable atambient temperatures; a liquid reservoir in fluid communication with oneof said at least one pressure vessel and containing a liquid that isunvaporizable in said liquid reservoir and in said at least one pressurevessel; and means for delivering the unvaporizable liquid from saidliquid reservoir to said one of said at least one pressure vessel,wherein the CWF is compressible within said one of said at least onepressure vessel upon direct contact with the unvaporizable liquid and isstorable in a liquid state after being compressed to its saturationpressure and condensed.

It is understood that the “unvaporizable liquid” may be able to bevaporized when subjected to other conditions, Under the conditionsimposed by the multiphase energy storage system, and particularly by theliquid reservoir and the at least one pressure vessel, the liquid, whichmay also be referred to as a “transfer liquid” is unvaporizable.

The CWF is preferably substantially isothermally compressible andexpandable during direct contact with the unvaporizable liquid withinsaid one of said at least one pressure vessel.

The CWF within said one of said at least one pressure vessel iscontinuously and additionally compressed while additional unvaporizableliquid is being introduced to said one of said at least one pressurevessel.

The energy storage system preferably further comprises at least onehydraulic turbine which is drivable by the unvaporizable liquid, whereinat least a portion of the unvaporizable liquid located within said oneof said at least one pressure vessel is propellable towards said atleast one hydraulic turbine by the compressed CWF. The unvaporizableliquid discharged from the at least one hydraulic turbine is receivablein the liquid reservoir.

In one aspect, the delivering means is at least one hydraulic pump fordelivering the unvaporizable liquid from the liquid reservoir to saidone of said at least one pressure vessel. The energy storage system mayfurther comprise at least one additional hydraulic pump for deliveringthe unvaporizable liquid from said one of said at least one pressurevessel to the liquid reservoir.

In one embodiment, the at least one pressure vessel includes first andsecond pressure vessels in fluid communication with each other, whereinsaid second pressure vessel is exposed to ambient temperatures, thesystem further comprising a gas source in fluid communication with saidfirst pressure vessel and containing the CWF, the CWF being feedablefrom said gas source to said first pressure vessel, wherein the CWF iscompressible within, and transferable from, said first pressure vesselupon direct contact with the unvaporizable liquid and is storable in aliquid state within said second pressure vessel after being transferredthereto and being compressed to its saturation pressure and condensed.

The CWF within the first pressure vessel is continuously andadditionally compressed while additional unvaporizable liquid is beingintroduced to the first pressure vessel.

The energy storage system preferably further comprises at least onehydraulic turbine which is drivable by the unvaporizable liquid, whereinat least a portion of the unvaporizable liquid located within the firstpressure vessel is propellable towards said at least one hydraulicturbine by the compressed CWF upon release from the second pressurevessel. The unvaporizable liquid discharged from the at least onehydraulic turbine is receivable in the liquid reservoir.

In one aspect, the delivering means is at least one hydraulic pump fordelivering the unvaporizable liquid from the liquid reservoir to thefirst pressure vessel. The energy storage may further comprise at leastone additional hydraulic pump for delivering the unvaporizable liquidfrom the first pressure vessel to the liquid reservoir.

In one aspect, the energy storage system further comprises a liquid-gasseparator and/or a liquid-liquid separator located between the first andsecond pressure vessels for preventing flow of the unvaporizable liquidto the second pressure vessel.

In one aspect, the first pressure vessel is also exposed to ambienttemperatures.

In one aspect, the energy storage system comprises a plurality of thefirst pressure vessels, wherein all of the first pressure vessels, orselected first pressure vessels, are in fluid communication with eachother. Alternatively, none of the plurality of first pressure vesselsare in fluid communication with each other.

In one aspect, the energy storage system comprises a plurality of thesecond pressure vessels, wherein all of the second pressure vessels, orselected second pressure vessels, are in fluid communication with eachother. Alternatively, none of the plurality of second pressure vesselsare in fluid communication with each other.

A method for producing power with stored energy, comprising the steps ofproviding a first pressure vessel and a second pressure vessel, whereinsaid second pressure vessel is capable of being in fluid communicationwith said first pressure vessel and is set to a temperature no greaterthan ambient temperatures; substantially isothermally compressing,within said first pressure vessel, a condensable working fluid (CWF)that is condensable at ambient temperatures during direct contact withan unvaporizable liquid and causing at least a portion of the compressedCWF to be transferred from said first pressure vessel to said secondpressure vessel in response to interaction with the unvaporizableliquid; transferring an additional amount of compressed CWF to saidsecond pressure vessel to cause all or a majority of the CWF within saidsecond pressure vessel to be compressed to its saturation pressure andbe condensed to produce liquid CWF; propelling at least some of theunvaporizable liquid located in said first pressure vessel by thecompressed CWF discharged from said second pressure vessel towards atleast one hydraulic turbine; and rotatably driving said at least onehydraulic turbine by the propelled unvaporizable liquid to producepower, wherein flow of the unvaporizable liquid to said second pressurevessel is prevented while the at least a portion of the compressed CWFis being transferred from said first pressure vessel to said secondpressure vessel.

As referred to herein, the meaning of the term “flow of theunvaporizable liquid to said second pressure vessel is prevented”includes the possibility that only a negligible volume of theunvaporizable liquid relative to the volume of the liquid CWF within thesecond pressure vessel is introduced to the second pressure vessel.Prevention of flow of the unvaporizable liquid to the second pressurevessel is carried out by suitable apparatus such as a demister.

In one aspect, the CWF is substantially isothermally compressed byactivating a unit of mixing equipment in fluid communication with thefirst or second pressure vessel and thereby eliminating temperaturegradients within the CWF.

In one aspect, the unit of mixing equipment is activated in response toa sensed condition indicative of liquefaction of the CWF.

In one aspect, the at least some of the unvaporizable liquid located insaid first pressure vessel is propelled by the compressed CWF dischargedfrom said second pressure vessel towards the at least one hydraulicturbine when the discharged compressed CWF is in a liquid state, a gasstate or in a multiphase state.

In one aspect, the step of causing at least a portion of the compressedCWF to be transferred from said first pressure vessel to said secondpressure vessel is performed during a plurality of charging cycles.

In one aspect, the step of propelling at least some of the unvaporizableliquid located in said first pressure vessel by the compressed CWFdischarged from said second pressure vessel is performed during aplurality of discharging cycles.

In one aspect, the method further comprises the steps of providing athird pressure vessel that is capable of being in fluid communicationwith said first pressure vessel, wherein, prior to performing one of acharging cycle or a discharging cycle, said third pressure vessel iscompletely filled with a first fluid selected from CWF gas or theunvaporizable liquid and said first pressure vessel is completely filledwith a second fluid selected from CWF gas or the unvaporizable liquidwhich is different than the first fluid; and performing the one of acharging cycle or a discharging cycle such that, when terminated, saidthird pressure vessel is completely filled with the second fluid andsaid first pressure vessel is completely filled with the first fluid.

In one aspect, the method further comprises performing the other of acharging cycle or a discharging cycle when said third pressure vessel iscompletely filled with the second fluid and said first pressure vesselis completely filled with the first fluid.

In one aspect, the compressed CWF discharged from said second pressurevessel expands substantially isothermally during direct contact with theunvaporizable liquid within the first pressure vessel.

A direct contact fluid transfer (DCFT) module, comprising a firstpressure vessel having at least four ports with each of which acorresponding conduit is in fluid communication, a second pressurevessel exposed to ambient temperatures which has one or more ports, anda flow control component operatively connected to each of saidcorresponding conduits for selectively controlling the flow therethroughof a fluid, wherein a condensable working fluid (CWF) which iscondensable at ambient temperatures is introducible to said firstpressure vessel through a first port of said at least four ports,wherein an unvaporizable liquid is introducible to said first pressurevessel through a second port of said at least four ports to causesubstantially isothermal compression of the CWF upon direct contact withthe unvaporizable liquid, wherein, following introduction of asufficient additional volume of the unvaporizable liquid to said firstpressure vessel, at least a portion of the compressed CWF istransferable from said first pressure vessel through a third port ofsaid at least four ports to said second pressure vessel through one ofsaid one or more ports upon direct contact with the unvaporizable liquidand is storable in a liquid state within said second pressure vesselafter being compressed to its saturation pressure and condensed, whereinat least some of the unvaporizable liquid located in said first pressurevessel is propellable through a fourth port of said at least four portsby the CWF, when discharged from said second pressure vessel, todischarge stored energy.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic illustration of an embodiment of a multiphaseenergy storage system;

FIG. 2 is a schematic illustration of another embodiment of a multiphaseenergy storage system;

FIG. 3 is a flow chart representing various steps that are involvedduring the performance of a cycle of the charging mode, according to oneembodiment;

FIG. 4 is a method for delivering gas by suction driven flow;

FIG. 5 is a flow chart representing various steps that are involved inthe performance of the discharging mode, according to one embodiment;

FIG. 6 is a schematic illustration of another embodiment of a multiphaseenergy storage system;

FIG. 7 is an exemplary pressure-volume diagram for the energy storagesystem of FIG. 1 ;

FIG. 8 is an exemplary temperature-entropy diagram for the energystorage system of FIG. 1 ;

FIG. 9 is a schematic illustration of another embodiment of a multiphaseenergy storage system, shown during charged conditions;

FIG. 10 is a schematic illustration of another embodiment of amultiphase energy storage system, showing operations performed during acharging cycle; and

FIG. 11 is a schematic illustration of the multiphase energy storagesystem of FIG. 10 , showing operations performed during a dischargingcycle.

DETAILED DESCRIPTION OF THE INVENTION

Significant heat losses experienced by the working fluid of prior artsystems are advantageously avoided in the multiphase compressed gasenergy storage system of the present invention by employing acondensable working fluid (CWF) that is normally gaseous but whichcondenses at ambient temperatures when being sufficiently compressed. Inconjunction with the CWF, a liquid based direct contact fluid transfer(DCFT) module is employed to ensure that the CWF will undergo phasechanges, whether in a charging mode or a discharging mode, substantiallyisothermally. The DCFT module is operable in conjunction with a transferliquid that is ensured in remaining in a liquid state, and thereforerelatively inexpensive hydraulic equipment interfacing with the flowingtransfer liquid is advantageously able to employed, to both reducesystem costs and increase the system round trip efficiency (RTE).

As referred to herein, “direct contact” means a heat transfer processthat involves the exchange of heat between two fluids which are broughtinto physical mutual contact when the two fluids are at differenttemperatures. Also, “substantially isothermally”, a condition wherebyheat transfer occurs at a near to constant temperature, is defined asless than or equal to 5 percent of the absolute temperature differentialbetween the final temperature and the initial temperature for anadiabatic heat transfer process having the same initial thermodynamicstate as the given heat transfer process. Any deviation from a purelyisothermal heat transfer process is due to a lower than required heattransfer coefficient, a smaller than required heat transfer area, or ashorter time than required for the heat transfer to take place.

FIG. 1 schematically illustrates an embodiment of a multiphase energystorage system, generally indicated as 1, that is operable in both acharging mode and a discharging mode. Two fluids interact in system 1,the first being a multiphase CWF fluid and the second being a transferliquid.

Multiphase energy storage system 1 comprises a gas holder 2 exposed toambient temperatures for holding low pressure CWF gas of close toatmospheric pressure. Gas holder 2 is an inexpensive large-volumecontainer, such as one delimited by a low strength membrane. Inaddition, system 1 comprises one or more compression tanks 4,constituting at least a part of the DCFT module, in fluid communicationwith gas holder 2, and one or more storage tanks 3 exposed to ambienttemperatures and in fluid communication with each compression tank 4,for storing liquid CWF of high energy density at the end of the chargingmode. The one or more compression tanks 4 may also be, non-limitingly,exposed to ambient temperatures. The one or more storage tanks 3 andcompression tanks 4 are preferably pressure vessels that can withstandthe relatively high pressure of compressed CWF gas. The transfer liquid,such as hydraulic oil or water, when held in liquid reservoir 5 may beexposed to air at atmospheric pressure and temperature to ensure thatthe transfer liquid will be maintained in liquid phase. Liquid reservoir5 is in fluid communication with each compression tank 4 and also withhydraulic supply pump 7 and hydraulic turbine 6 used for exploiting thestored gas energy in the discharging mode. The transfer liquid isoptionally immiscible with the CWF fluid.

As shown in FIG. 2 , costs for operating multiphase energy storagesystem 1 may be reduced by powering supply pump 7 as well as a returnpump, if used, during the charging mode with electrical energy generatedfrom a renewable energy source 11, such as a photovoltaic power systemor a wind farm, or from any other energy source such as a power plant.The electrical power produced by renewable energy source 11 is generallysupplied to electrical grid 14. When the power demand is lower than thepower produced by the renewable energy source, the excess power can bedelivered to the pumps to charge the system. An excess amount ofelectrical power produced by renewable energy source 11 may be used topower the supply and return pumps. Likewise, the electrical powerproduced by a generator coupled to hydraulic turbine 6 may be suppliedto electrical grid 14. The system may also be powered by other meanswell known to those skilled in the art under certain conditions.

FIG. 6 illustrates another embodiment of a multiphase energy storagesystem, generally indicated as 21. System 21 is similar to system 1, andis configured with additional apparatus, such that each differentcomponent, such as gas discharge conduit 23, mixing equipment 24,liquid-gas separator 26, and heat exchanger 28, constitutes a furtherembodiment that provides advantageous features. With respect to energystorage system 21, liquid reservoir 25 is not exposed to the surroundingair, but rather is in fluid communication with gas holder 2 by gasdischarge conduit 23, allowing any gas that has been de-gassed from thetransfer liquid, after having been absorbed therein during the chargingmode or discharging mode, to flow in return to the gas holder by aclosed conduit circuit arrangement. The transfer liquid remainsunvaporizable under the given thermodynamic conditions of the chargingmode and discharging mode whereby the pressure of the transfer liquid issignificantly higher than its saturation pressure for the controlledtemperature range.

FIG. 9 illustrates another embodiment of a multiphase energy storagesystem, generally indicated as 71. The cost effective system 71comprises a single compression tank 74 exposed to atmospherictemperatures within which CWF is retained and is able to be condensedwhile being directly contacted by the unvaporizable transfer liquid,without need of a storage tank. Liquid reservoir 5 adapted to receivethe transfer liquid is exposed to atmospheric air and pressure, and isin liquid communication with the liquid feed conduit to which hydraulicpump 7 is operatively connected and with the turbine inlet conduit towhich hydraulic turbine 6 is operatively connected. System 71 may alsointerface with renewable energy source 11 and electrical grid 14, asdescribed with respect to FIG. 2 .

A supplementary gas tank (not shown) may be used to inject asupplementary amount of gas to a compression tank or to a gas holder ifthe current mass of gas retained in system 1 of FIG. 1 or system 71 ofFIG. 9 is less than a minimal value needed for an efficacious operationfor producing power from stored gas energy.

Charging Mode

Initially, with reference to multiphase energy storage system 71 of FIG.9 , the transfer liquid is retained in liquid reservoir 5, low-pressureCWF is retained in compression tank 74 and all valves are closed. Afterthe liquid feed valve is opened and supply pump 7 is activated, transferliquid is delivered to compression tank 74. The introduced transferliquid reduces the available volume within compression tank 74 that theCWF is able to occupy, causing the CWF to become compressed. Asadditional transfer liquid is introduced, the CWF becomes increasinglycompressed until its pressure is increased above its saturation pressureand condenses. The CWF is compressed substantially isothermally as aresult of the direct contact with the transfer liquid. Upon conclusionof the charging mode, the liquid feed valve is closed and the supplypump is deactivated.

In pre-charging conditions with reference to system 1 of FIG. 1 andsystem 21 of FIG. 6 , the CWF is retained in gas holder 2 in gas phase,being at atmospheric pressure or a pressure slightly thereabove, thetransfer liquid is retained in liquid reservoir 5, and the one or morestorage tanks 3 are precompressed with gas, and the one or morecompression tanks 4 are filled with atmospheric gas or are filled withthe transfer liquid.

If the one or more compression tanks 4 are filled with transfer liquid,the procedure described in relation to FIG. 4 is performed to urgedelivery of the transfer liquid to liquid reservoir 5 while gas flowsfrom gas holder 2 to one or more compression tanks 4.

When there is a sufficient volume of gas within the one or morecompression tanks 4, the isolation valve 3B at the at least one port toeach storage tank 3 is opened, to provide a combined interior volumethat is common to each compression tank 3 and storage tank 4 viaterminal conduit 3A extending therebetween. The gaseous CWF flows fromeach compression tank 4 through terminal conduit 3A is received in astorage tank 3 after flowing through isolation valve 3B without beingdischarged therefrom. If there are more than one storage tank 3, thegaseous CWF flows in parallel through a corresponding isolation valve 3Band port to the storage tank, although not all isolation valves 3B maybe open at the same time.

Following introduction of a first portion of the CWF gas to the one ormore tanks 3 and 4, liquid feed valve 1B operatively connected to aregion of liquid feed conduit 1A extending from liquid reservoir 5 tothe one or more compression tanks 4 that is downstream to supply pump 7which is also operatively connected to liquid feed conduit 1A is opened.Hydraulic supply pump 7 is activated, and is caused to deliver thetransfer liquid from reservoir 5 to the one or more compression tanks 4.

The transfer liquid that is being introduced to the one or morecompression tanks 4 reduces the combined volume of tanks 3 and 4 that isoccupied by the CFW gas. Consequently the CFW gas is able to becompressed within the interior of a compression tank 4. As more transferliquid is introduced to the one or more compression tanks 4, thecombined volume of tanks 3 and 4 that is occupied by the CFW gas isreduced and the CFW in both gas tanks 3 and 4 becomes additionallycompressed. During compression, the CWF gas is directly contacted andcooled by the transfer liquid to reduce the heat of compression, suchthat the CWF gas is able to undergo compression at a substantiallyconstant temperature. Eventually, the one or more compression tanks 4are completely occupied by the transfer liquid while the CWF gas thatwas formerly in the one or more compression tanks 4 is displaced to theone or more storage tanks 3, causing the CFW gas within the one or morestorage tanks 3 to be further compressed. The CWF received in the one ormore storage tanks 3 liquefies once it is compressed to at leastsaturation pressure. All valves are then closed to complete a cycle ofthe charging mode.

If the pressure of the CWF received in the one or more storage tanks 3is less than saturation pressure, additional cycles of the charging modemay be performed. During each additional cycle, another gas portion isfed to a compression tank 4 to increase the pressure of the stored CWFfluid.

FIG. 3 illustrates various steps that are involved in the performance ofa cycle of the charging mode in conjunction with system 1 of FIG. 1 andsystem 21 of FIG. 6 , according to one embodiment.

To initiate an additional cycle of the charging mode, or alternatively afirst cycle thereof, when the one or more compression tanks 4 is filledwith the transfer liquid in step 29, return valve 5B operativelyconnected to another conduit 5A extending from liquid reservoir 5 to theone or more compression tanks 4, i.e. the return conduit, is opened instep 31 to cause the transfer liquid to be delivered from the one ormore compression tanks 4 in return to reservoir 5. At the same time, gasfeed valve 4B is momentarily opened to cause flow of a gas portion fromthe gas holder 2 to the one or more compression tanks 4.

All valves are closed once the transfer liquid has returned to reservoir5. Afterwards, each isolation valve 3B is opened in step 36. Supply pump7 is then activated in step 37 to supply the one or more compressiontanks 4 with transfer liquid, so that the CWF gas that has been fed fromthe gas holder to the one or more compression tanks 4 in the currentcycle will be compressed in step 39 and subsequently displaced in step41 to the one or more storage tanks 3 by the transfer liquid. Theintroduction of additional CWF gas to the one or more storage tanks 3increases the pressure of the stored CWF gas in step 43. Eventuallyafter one or more cycles, the pressure of the ambient-temperature CWFreceived in the one or more storage tanks 3 increases to the saturationpressure and then liquefies in step 45.

If the one or more compression tanks 4 is filled with gas in step 35,the charging mode cycle proceeds to step 36.

In another embodiment, gas is fed to the one or more compression tanksvia the gas feed valve in step 33 and steps 37 and 39 are performed tocompress the fed gas within the one or more compression tanks while theisolation valve at the inlet port of each storage tank is closed toincrease the pressure that is building up in a compression tank. Afteran isolation valve is then opened, the compressed fluid is displaced bythe transfer liquid to a storage tank. The displaced fluid may be in agaseous state or a two-phase liquid-vapor state, undergoing first stagecompression in a compression tank and second stage compression in astorage tank until completely liquefying.

In another embodiment shown in FIG. 4 , a portion of gas is drawn fromthe gas holder by suction, if for example the pressure differentialbetween the gas holder and compression tank is relatively low. Suchsuction driven flow is made possible when the one or more compressiontanks are filled with transfer liquid in step 31. While the gas feedvalve is opened, the return pump operatively connected to the returnconduit extending between the one or more compression tanks and thereservoir is activated in step 44. The flow of the transfer liquidthrough the return conduit in the direction of the reservoir generates asuction that draws gas from the gas holder to the one or morecompression tanks in step 48.

The volume of CWF fluid at a given pressure that can be stored when in aliquid state is significantly smaller than when provided in a gaseousstate, and therefore the ability of systems 1, 21 and 71 to store theCWF in a liquid state significantly increases the energy density, i.e.the potential stored energy per volume, of the working fluid relative tothe prior art practice of storing the working fluid in a gaseous state.The energy density achievable by the CWF is also able to beadvantageously increased by virtue of the substantially isothermalcompression and expansion, which results in reduced heat loss. Thecapital cost of the small-volume storage tanks 3 is substantially lessthan the large-volume storage tanks that are needed to store compressedgas in a gaseous state.

The energy density of the fluid stored in the one or more storage tanks3 may be additionally increased, as shown in system 21 of FIG. 6 , whengas holder 2 contains an additional low-pressure gas which is a non-CWFbut which is mixable with the CWF, such as air or a non-condensable gas(NCG). This mixture is compressed in step 39 and transferred to the oneor more storage tanks in step 41. After the CWF is liquefied in step 45,the non-CWF gas is able to be additionally compressed, such as inresponse the introduction of an additional volume of the transfer liquidto a compression tank 4, to further increase the energy density of thestored fluid. It will be appreciated that a non-CWF may also be mixedwith the CWF to increase the energy density of the stored compressedfluid in systems 1 and 71.

The high energy density of the CWF received in the one or more storagetanks 3 is advantageous since more energy is able to be released fromthe stored CWF in the discharging mode.

With reference to multiphase energy storage system 21 schematicallyillustrated in FIG. 6 according to another embodiment, the energydensity of the CWF transferred to a storage tank 3 is able to be furtherincreased by preventing the flow of the transfer liquid into eachstorage tank, so that more CWF is able to occupy a storage tank interiorand to be condensed. The flow of transfer liquid into a storage tank maybe prevented by deploying a liquid-gas separator 26, or a liquid-liquidseparator when the CWF condenses as well within a compression tank, atthe inlet of the storage tank port. Liquid-gas separator 26 may be forexample one that forces the transfer liquid to fall under the influenceof gravity within the liquid-gas separator and return to a compressiontank 4. Liquid-gas separator 26 may also be embodied by a buffer tanklocated above the height of each compression tank 4. Liquid-gasseparator 26 or a liquid-liquid separator may be operatively connectedto conduit 27, which extends from a dedicated outlet provided with acorresponding compression tank 4 to terminal conduit 3A with which it isin fluid communication.

Despite the lack of direct contact between the transfer liquid and theCFW fluid within the one or more storage tanks 3 when systems 1 and 21employ a liquid-gas separator 26, the CFW is able to be isothermallycompressed as a result of the cooling effect provided by theambient-temperature air or ground surrounding the one or more storagetanks 3. Additionally, the heat of compression is able to be reduced oraltogether eliminated when the CFW is slowly compressed, for example onthe order of several hours.

Alternatively or additionally, as schematically illustrated inmultiphase energy storage systems 21 of FIGS. 6 and 71 of FIG. 9 ,isothermal compression may be ensured by means of mixing equipment. Aunit 24B of mixing equipment set in fluid communication with acorresponding storage tank 3 may be activated when the CWF starts toliquefy, such as when achieving a two-phase liquid-vapor state. Theliquid and vapor portions of the CWF will be able to be mixed togetherto eliminate temperature gradients within the stored CWF by providing afluid with a substantially uniform temperature and heat transfercoefficient, and also absorbing any released heat of compression. Unit24B may be activated in response to the detection of a predeterminedpressure within the corresponding storage tank 3 or the detection of thepresence of liquid, by a relevant sensor. Likewise a unit 24A of mixingequipment, which may be identical to or different from unit 24B, may beset in fluid communication with a corresponding compression tank 4.

The mixing equipment may assume various forms. The mixing equipment,such as an agitator, may be rotatably mounted with the correspondingtank. An agitator provides homogeneity of the stored fluid in terms oftemperature, heat transfer coefficient, as well as in terms ofcomposition to mix any particles that may have settled out of thecompressed liquefied solution.

The mixing equipment may also be deployed externally to thecorresponding tank. In one embodiment, the mixing equipment isconfigured with a set of conduits and nozzles, whereby a fluid portionat one region of the tank is bled through one of the conduits, such asin conjunction with a pump, and is injected to another region of thetank through one or more nozzles. This arrangement may also facilitatecirculation of the transfer liquid between two or more of thecompression tanks 4, which may be interconnected, or from one or more ofthe compression tanks 4 to one or more of the storage tanks 3, which mayalso be interconnected.

Also, a heat exchanger 28A set in heat exchanger relation with the oneor more compression tanks 4 or a heat exchanger 28B set in heatexchanger relation with the one or more storage tanks 3 may be providedto assist in achieving isothermal compression. Each of the heatexchangers may be of the shell and tube type, finned tube type, plateheat exchanger type, air cooled type, or any other heat exchanger typewell known to those skilled in the art. The tubes through which thecooling medium flows, when employed, may be shared with the conduitsassociated with the mixing equipment, such as when one or more valvesare selectively opened and/or closed to permit flow to a feat exchangeror to a mixing equipment unit. More than one heat exchanger may beemployed, and each one may be of the same type or of a different type.

Discharging Mode

The condensed CWF is able to remain stored for a prolonged period oftime while being subjected to the charged conditions until thedischarging mode is initiated. The discharging mode is generallyinitiated during peak demand or during periods when there is a shortagein energy availability as the energy stored in the multiphase energystorage system is able to be discharged whenever desired and to produceelectricity supplied to the grid.

During the discharge mode, transfer liquid is propelled by high-pressureliquid CWF towards the hydraulic turbine in order to produceelectricity. By using a transfer liquid for causing compression of theCWF during the charging mode and for being propelled by the CWF duringthe discharging mode, a hydraulic turbine may advantageously be employedto produce power, as opposed to a gas turbine used by prior artcompressed gas storage systems. Advantages of the use of a hydraulicturbine relative to a gas turbine include higher efficiencies, lowerrotational speeds, easier maintainability and lower running costs.

These advantages are also relevant to the use of a hydraulic pump by themultiphase energy storage system as opposed to the use of a compressorfor introducing compressed gas by a prior art compressed gas storagesystem.

Referring now to multiphase energy storage system 71 of FIG. 9 , CWFliquid and high-pressure transfer liquid are retained within compressiontank 74 in anticipation of the subsequent initiation of the dischargingmode. Since the CWF fluid has been compressed to its saturationpressure, which is significantly greater than its pressure duringpre-charging conditions, and transfer liquid is retained in the sameclosed compression tank 74 as the CWF liquid, the CWF liquid applies aforce onto the transfer liquid that causes the transfer liquid to bepressurized to the same saturation pressure as the CWF liquid. Thepressurized transfer liquid located within compression tank 74 is at asignificantly higher pressure than the atmospheric pressure to which thetransfer liquid retained within liquid reservoir 5 is exposed.

When the turbine injection valve is opened to initiate the dischargingmode, the high-pressure transfer liquid is urged to be discharged fromcompression tank 74 with sufficiently high kinetic energy to rotatablydrive hydraulic turbine and produce power due to the high pressuredifferential between the high-pressure transfer liquid withincompression tank 74 and the low-pressure transfer liquid within liquidreservoir 5. In response to the discharge of high-pressure transferliquid from compression tank 74, additional volume of compression tank74 that is unoccupied by the transfer liquid is made available to theCWF, which is consequently able to expand and to achieve a two phasevapor-liquid state. It is noted that since the multiphase CWF expandsisobarically, the pressure of the CWF and of the transfer liquidpressurized thereby is unchanged, thereby allowing the power output tobe continuous for a period of time. Once the CWF achieves a gaseousstate, the pressure of the high-pressure transfer liquid is reduced.Nevertheless, the pressure of the propelled transfer liquid, untilreduced below a turbine-specific threshold, is sufficiently high torotatably drive hydraulic turbine 6 and to produce power. At the end ofthe discharging mode, the turbine injection valve is closed and theliquid feed valve is opened.

By virtue of the simplicity of system 71, transfer liquid is able to becycled back and forth between liquid reservoir 5 and compression tank 74during alternate performance of the charging and discharging modes,respectively, in a similar fashion as a piston to produce additionalpower.

In pre-discharge conditions with reference to system 1 of FIG. 1 andsystem 21 of FIG. 6 , all valves are closed and the CWF fluid retainedin the one or more storage tanks 3 is in liquid phase, after having beenexposed to the ambient temperature of the storage tanks and beencompressed to a relatively high pressure. If mixed together with the CWFfluid, non-CWF gas compressed to a higher pressure than that of the CWFliquid is also retained within the one or more storage tanks 3.

FIG. 5 illustrates various steps that are involved in the performance ofthe discharging mode, according to one embodiment.

In a first step 51 of the discharging mode, return valve 5B ismomentarily opened, allowing transfer liquid received in reservoir 5 ofa relatively high hydraulic head to flow through return conduit 5B andfill the one or more compression tanks, if no transfer liquid iscurrently contained within any of the compression tanks 4 following thefinal cycle of the charging mode. Alternatively, liquid feed valve 1Bmay be momentarily opened and supply pump 7 may be operated to feed arequired volume of transfer liquid to the one or more compression tanks4. This step may be dispensed with if there is a sufficient volume oftransfer liquid within the one or more compression tanks 4. Each of thecompression tanks may be equipped with a sensor, such as a capacitancelevel sensor or a float switch, to determine the presence of apredetermined level of transfer liquid, and possibly also with apressure sensor. Return valve 1B or valve 5B is then closed to preventreturn flow of the transfer liquid.

In step 53, turbine injection valve 2B and at least one storage tankisolation valve 3B are opened. The high-pressure compressed fluidincluding at least CWF liquid will consequently be released from astorage tank 3 in step 55 via terminal conduit 3A with which acorresponding opened isolation valve 3B is in fluid communication due tothe exposure to the lower pressure in compression tank 4 and expand to asmall degree. The released CWF liquid forcefully contacts the transferliquid in step 57 within each compression tank 4 and, as a result of thepressure differential between the CWF liquid and the transfer liquid inthe liquid reservoir, causes the transfer liquid to be propelled throughturbine inlet conduit 2A to which turbine injection valve 2B isoperatively connected. The transfer liquid is able to be propelled aslong as the pressure differential between the transfer liquid that ispropelled by the CWF, which is at the same pressure as the CWF, and thetransfer liquid in liquid reservoir 5 downstream to the hydraulicturbine 6 is greater than a predetermined threshold. The CWF liquid isable to evaporate and achieve a two-phase liquid-vapor state while beingisobarically expanded in step 59 as a result of the greater volumewithin the combined interior of tanks 3 and 4 that it occupies inresponse to the displacement of the transfer liquid. The pressure of themultiphase CWF fluid and also of the CWF when in an entirely gaseousphase is sufficiently high to continue propelling the transfer liquidthrough turbine inlet conduit 2A. By virtue of the direct contactbetween the CWF gas and the transfer liquid, a change in temperature ofthe CWF gas is reduced, allowing the CWF gas to expand substantiallyisothermally. This expansion process continues while the transfer liquidis increasingly propelled and the CWF fluid which achieves a completelygaseous state occupies an increased volume. The transfer liquidpropelled through turbine inlet conduit 2A consequently rotatably driveshydraulic turbine 6 in step 61 to produce power. The transfer liquidexiting hydraulic turbine 6 flows to reservoir 5 in step 63 to increasethe liquid level within the liquid reservoir. All valves are closed tocomplete a cycle of the discharge mode in step 65 when the volume of thetransfer liquid remaining in the one or more compression tanks is lessthan a propelling-worthy volume. Afterwards, gas feed valve 4B is openedin step 67, causing the gas remaining in the one or more compressiontanks to flow back to gas holder 2 through gas feed conduit 4A due to apressure differential between the one or more compression tanks 4 andthe gas holder 2.

Thus the use of the phase changing CWF fluid during the discharging modedoes not reduce the amount of power that is able to be extracted, butrather the multiphase CWF fluid is involved in continuing to propel thetransfer liquid towards the hydraulic turbine to produce an additionalamount of power. This power extraction ability is in addition to theadvantage provided by the condensing fluid whereby the volume of thecompression and storage tanks is allowed to be reduced.

One or more cycles involving the steps above may be repeated in thedischarge mode if the pressure of the fluid including at least CWF thatremains in the one or more storage tanks 3 is sufficiently greater thanatmospheric pressure to facilitate propulsion of the transfer liquid.That is, step 51 is performed to refill the one or more compressiontanks 4 with transfer liquid from reservoir 5 if they have becomesufficiently emptied, and then the refilled transfer liquid is propelledin step 57 by the fluid released from the one or more storage tanks.Additional cycles may be repeated until the pressure of the gas withinthe one or more storage tanks 3 is lowered in step 69 to a pressurebelow a propelling-worthy pressure at which the transfer liquid iscapable of being propelled.

The sequence of the various steps of both the charging and dischargingmodes is able to be controlled manually upon manipulating the variousflow control devices, or alternatively automatically in response to theoperation of a controller. Each step may be performed in response to atimed or sensed action commanded by the controller. Through theintervention of the controller, a charging step and a discharging stepmay be synchronized to provide a continuous discharging operation.

By employing a large-volume gas holder 2 and by performing a pluralityof charging and discharging cycles, the volume of the one or morecompression tanks 4 is advantageously able to be reduced, for lowercosts. Depending on the gas energy storage need, systems 1 and 21 can bescaled up or down by a change in the number or size of one or moretanks. When system 1 or system 21 comprises a plurality of storage tanks3 and compression tanks 4, each of the storage tanks and each of thecompression tanks may be interconnected. For a more efficient transferoperation, a different step may be performed simultaneously inconjunction with two different tanks. For example, liquid transfer canflow to a first compression tank 4 at the start of another dischargingcycle, while at the same time in a second compression tank the liquidtransfer is propelled by expanding CWF in an a different dischargingstep to rotatably drive the hydraulic turbine.

FIGS. 7 and 8 illustrate various exemplary thermodynamic states of theCWF fluid. At state F, CWF gas is introduced to a compression tank, forexample from the gas holder, at a relatively low pressure, and thepressure of the CWF gas is steadily increased in response to theintroduction of transfer liquid into the compression tank, to thesaturation pressure of the CWF at state G. The CWF, after undergoing aphase change at state G to assume a two-phase fluid, is isobaricallycondensed until being completely liquefied to occupy a minimal volume atstate H. By direct contact with the transfer liquid and influence of thesurroundings, the CWF is isothermally compressed throughout the chargingmode from state F to state H. These conditions are reversed when the CWFfluid is isothermally expanded throughout the discharging mode fromstate H to state F.

Although only a single hydraulic turbine 6 is shown is FIGS. 1, 6 and 9, it is appreciated that more than one hydraulic turbine may beemployed. When the pressure of the CWF propelling the transfer liquid issufficiently high as detected by sensors, more than one hydraulicturbine stage may be used to produce an additional amount of power. Adifferent turbine type may be used for each stage in order to maximizeefficiency with respect to the designed operating conditions of aspecific stage, such as a predetermined range of pressure drop, specificspeed or head. Alternatively, a plurality of turbines in parallel may beemployed. A controller receiving pressure readings as detected bysensors is able to command a flow control component such as a valve atthe outlet of a first stage turbine to urge delivery of the transferliquid exiting the first stage turbine to the inlet of a second stageturbine and to rotatably drive the second stage turbine.

It will be appreciated that a plurality of supply pumps in parallel orin line may be used.

As described hereinabove, a plurality of charging and discharging cyclesare able to be alternately performed with respect to an energy storagesystem, a charging cycle being repeatable if the CWF pressure is lessthan saturation pressure and a discharging cycle being repeatable if thepressure of the propelling fluid including at least CWF is sufficientlygreater than atmospheric pressure to facilitate propulsion of thetransfer liquid.

FIGS. 10 and 11 illustrate another embodiment of a multiphase energystorage system, generally indicated as 91. System 91 is a closed systemthat comprises two or more DCFT modules that are each capable of beingin fluid communication with the gas holder, the one or more storagetanks, the hydraulic turbine and at least another DCFT module. By beingprovided with two or more DCFT modules, charging or discharging cyclesare advantageously able to be repeatedly performed with a minimalwaiting time between subsequent cycles.

Multiphase energy storage system 91 comprises a gas holder 92 exposed toambient temperatures, which is shown to hold low pressure CWF gas ofclose to atmospheric pressure as well as an additional low-pressurenon-CWF gas, such as air or an NCG, but which may hold only CWF. Inaddition, system 91 comprises two or more DCFT modules, e.g. DCFTmodules 93 a and 93 b, one or more storage tanks, e.g. three storagetanks 94 a-c, exposed to ambient temperatures for storing CWF,particularly liquid CWF or a mixture of liquid CWF and NCG of highenergy density at the end of the charging mode, at least one hydraulicturbine 96, low pressure liquid reservoir 98 for receiving the liquiddischarged from hydraulic turbine 96, and pumps 108 and 109. The DCFTmodules 93 a-b, which may be referred to as the “first module” and“second module”, respectively, are preferably pressure vessels that canwithstand the relatively high pressure of compressed CWF gas. The fluidretained by gas holder 92 and flowable within the energy storage systemmay also be referred to as “CWF-based fluid”. A schematicallyillustrated controller 107 in data communication with at least pumps 108and 109 and the valves synchronizes flow of the transfer liquid and ofthe CWF-based fluid.

FIG. 10 illustrates operation of system 91 during the charging mode. Inpre-charging conditions, all valves are closed. Also, the first DCFTmodule 93 a is filled with the unvaporizable transfer liquid, and thesecond DCFT module 93 b, as well as storage tanks 94 a-c, are filledwith CWF or a mixture of CWF and non-CWF gas, which is maintained in agas phase and at a predetermined pre-compressed pressure prior to beingcompressed by the transfer liquid. First DCFT module 93 a, second DCFTmodule 93 b, and storage tanks 94 a-c may be prefilled by well-knownmeans such as with a pump and valve, or by any other suitable means.

Pressure control means 103B connected to second DCFT module 93 b, whichmay be in data communication with the controller, ensures that theinterior of the second module achieves the predetermined pre-compressedpressure. Pressure control means 103B may be embodied by means wellknown to those skilled in the art such as a pressure gauge and apressure regulator, and optionally may also comprise an isothermalcompressor for quickly increasing the gas pressure within the interiorof the second module while the heat of compression is minimized. Similarpressure control means 103A may be connected to first DCFT module 93 a.

To initiate a first charging cycle, first module liquid discharge valve101A, second module liquid inlet valve 102B, liquid feed valve 114,first module gas inlet valve 111A, second module gas inlet valve 111B,and second module gas transfer valve 117B are opened. As second modulegas inlet valve 111B is opened and is in fluid communication with gasholder 92, the pressure of gas within the gas holder is also maintainedat the predetermined pre-compressed pressure. First pump 108 is thenactivated and the transfer liquid is delivered from first module 93 a tosecond module 93 b, flowing in the direction of arrow A through conduit121 extending from a first port of the first module, through conduit 123with which first pump 108 and liquid feed valve 114 are operativelyconnected, and in the direction of arrow B through conduit 124 extendingfrom the end of conduit 123 to a second port of second module 93 b.

The transfer liquid that is being introduced to second module 93 breduces the volume therewithin that is occupied by the retained gas.Consequently the retained gas is able to be compressed within theinterior of second module 93 b. During compression, the retained gas isdirectly contacted and cooled by the transfer liquid to reduce the heatof compression, such that the retained gas is able to undergosubstantially isothermal compression. When second module 93 b is beingfilled with the transfer liquid, the compressed CWF-based gas is urgedto be displaced and to flow across a sixth port thereof, flowing in thedirection of arrow C through conduit 127 and of arrow D through conduit128 and the corresponding conduits 129 to a first port of the one ormore storage tanks 94 a-c. The compressed gas transferred from secondmodule 93 b causes the stored CWF-based gas in each of the one or morestorage tanks 94 a-c to become additionally compressed.

Substantially isothermal compression of the CWF-based fluid may beensured by circulating cooling transfer liquid located at the bottom ofsecond module 93 b by means of second pump 109 in a closed loop acrossfourth and fifth ports of the second module and through conduits136-138, when isolation valve 116B operatively connected to conduit 136and isolation valve 118B operatively connected to conduit 138 areopened. The degree of cooling provided by the circulating transferliquid may be increased by means of a heat exchanger 104 in heatexchanger relation with conduit 137. Alternatively or additionally,mixing equipment or spray nozzles in use when isolation valves 116B and118B are opened are able to assist in ensuring substantially isothermalcompression.

In response to the pumped discharge of transfer liquid from first module93 a, a portion of the CWF-based gas retained by gas holder 92 flows inthe direction of arrows E and F to first module 93 a via conduits131-133 and a third port of the first module, maintaining the non-liquidvolume within the first module at the pre-compressed pressure inanticipation of a subsequent charging cycle.

The first charging cycle is terminated when second module 93 b iscompletely filled with transfer liquid, first module 93 a is filled withCWF-based gas at the pre-compressed pressure, and the one or morestorage tanks 94 a-c are completely filled with CWF-based gas, i.e. noother fluid is received in the one or more storage tanks, at a higherpressure than the pre-compressed initial pressure, after having beencompressed by the CWF-based gas transferred thereto from second module93 b. All valves are then closed.

To initiate a second charging cycle, second module liquid dischargevalve 101B, first module liquid inlet valve 102A, liquid feed valve 114,first module gas inlet valve 111A, second module gas inlet valve 111Band first module gas transfer valve 117B are opened. First pump 108 isthen activated and the transfer liquid is delivered from second module93 b to first module 93 a, flowing in the direction of arrow G throughconduit 141 extending from a first port of the second module, throughconduit 123, and in the direction of arrow H through conduit 144extending from the end of conduit 123 to a second port of first module93 a. The same process performed in the first charging cycle is repeatedin the second charging cycle. Thus when first module 93 a is filled withthe transfer liquid, the compressed gas is urged to be displaced and toflow across a sixth port thereof, flowing in the direction of arrow Ithrough conduit 147 and of arrow D through conduit 128 and thecorresponding conduits 129 to the one or more storage tanks 94 a-c. Thecompressed gas transferred from first module 93 a causes the storedCWF-based gas in each of the one or more storage tanks 94 a-c to becomeadditionally compressed.

The second charging cycle is terminated when first module 93 a iscompletely filled with transfer liquid, second module 93 b is filledwith CWF-based gas at the pre-compressed pressure, and the one or morestorage tanks 94 a-c are completely filled with CWF-based fluid at ahigher pressure than the pressure achieved at the end of the firstcharging cycle, after having been compressed by the CWF-based gastransferred thereto from first module 93 a. All valves are then closed.

The charging cycles are likewise able to be repeated, insofar as thefirst and second modules are alternately filled with transfer liquid andCWF-based gas at the pre-compressed pressure, respectively, at eachsubsequent charging cycle. By delivering the transfer liquid back andforth between the first and second modules during each subsequentcharging cycle and supplementing the volume of the module from whichtransfer liquid was discharged with CWF-based gas from the gas holder,the two modules are immediately ready for use in a subsequent chargingcycle upon termination of the previous charging cycle. The chargingprocess will continue until the CWF-based fluid in the one or morestorage tanks 94 a-c will be condensed and change phase from gas toliquid. The liquid CWF-based fluid is stored in the one or more storagetanks 94 a-c until discharge is required.

During any of the charging cycles, if required, the stored CWF-basedfluid is able to undergo a cooling operation, a heating operation or amixing operation in conjunction with suitable apparatus 122 upon openingisolation valve 119 operatively connected to conduit 126, the latterbeing in fluid communication with second and third ports of each of thestorage tanks 94 a-c in parallel to facilitate circulation of the storedCWF-based fluid by a closed loop. Alternatively, conduit 126 is adaptedto deliver cooling medium such as water that is able to flow within acooling coil provided internally to the one or more storage tanks.

FIG. 11 illustrates operation of system 91 during the discharging mode.In pre-discharge conditions, all valves are closed. Also, one module iscompletely filled with transfer liquid and the other module iscompletely filled with CWF-based gas at the pre-compressed pressure.With respect to the following exemplary first discharging cycle, firstmodule 93 a is completely filled with transfer liquid and second module93 b is completely filled with CWF-based gas at the pre-compressedpressure.

To initiate a first discharging cycle, first module liquid dischargevalve 101A, first module gas transfer valve 117A, and turbine injectionvalve 151 are opened. A predetermined amount of CWF-based fluid, whichmay be controlled by momentarily opening first module gas transfer valve117A for a predetermined duration or in response to a sensed value, isreleased from the one or more storage tanks 94 a-c, flowing throughconduits 129, 128 and 147 in directions J and K until being introducedwithin the interior of first module 93 a. The released CWF-based fluidforcefully contacts the transfer liquid within first module 93 a so thatthe transfer liquid will become pressurized to the same pressure as thereleased CWF-based fluid. As a result of the pressure differentialbetween the turbine inlet, whose pressure is substantially equal to thatof the pressurized transfer liquid, and the turbine outlet which isexposed to the low-pressure reservoir 98, the transfer liquid is causedto be propelled through conduits 121, 148 and 153 in directions L and Mtowards hydraulic turbine 96. Conduit 153 with which turbine injectionvalve 151 is operatively connected extends from the junction betweenconduits 141 and 148 to reservoir 98. The transfer liquid driveshydraulic turbine 96 to generate electricity which is able to besupplied to the grid. Reservoir 98, which may be in fluid communicationwith gas holder 92 via interconnecting conduit 143 and is consequentlyunexposed to the environment, or alternatively is in fluid communicationwith the surrounding air, receives the transfer liquid discharged fromhydraulic turbine 96.

In response to the discharge of high-pressure transfer liquid from firstmodule 93 a, additional volume of first module 93 a that is unoccupiedby the transfer liquid is made available to the CWF-based fluid, whichis consequently able to expand and to achieve a two phase vapor-liquidstate or a completely gaseous phase. By virtue of the direct contactbetween the CWF-based fluid and the transfer liquid, a change intemperature of the CWF-based fluid is reduced, allowing the CWF-basedfluid to expand substantially isothermally.

Substantially isothermal expansion of the CWF-based fluid within firstmodule 93 a, during which the CWF-based fluid undergoes a coolingprocess, may be ensured by circulating heating transfer liquid locatedat the bottom of first module 93 a by means of second pump 109 in aclosed loop across fourth and fifth ports of the first module andthrough conduits 135, 137 and 139, when isolation valve 116A operativelyconnected to conduit 135 and isolation valve 118A operatively connectedto conduit 139 are opened. Alternatively or additionally, mixingequipment or spray nozzles in use when isolation valves 116A and 118Aare opened are able to assist in ensuring substantially isothermalexpansion.

Simultaneously to the discharge of high-pressure transfer liquid fromfirst module 93 a, second module liquid inlet valve 102B, second modulegas outlet valve 113B and return valve 156 are opened. Thus, while thetransfer liquid is flowing into reservoir 98, the received transferliquid is delivered by first pump 108 to second module 93 b inanticipation of another discharge cycle, flowing through return conduit158 and conduits 123 and 124 in directions N and O. In response,CWF-based gas is displaced by the delivered transfer liquid from aseventh port of second module 93 b to gas holder 92 through conduits 131and 149 in directions P and Q. The first discharging cycle is terminatedwhen first module 93 a is filled completely with CWF-based gas at apredetermined low pressure and second module 93 b is filled completelywith transfer liquid at near-atmospheric pressure. All valves are thenclosed.

To initiate a second discharging cycle, second module liquid dischargevalve 101B and turbine injection valve 151 are opened. When secondmodule gas transfer valve 117B is momentarily opened, during apredetermined duration or until a sensed value is detected, apredetermined amount of CWF-based fluid is released from the one or morestorage tanks 94 a-c, flowing through conduits 127-129, 128 indirections J and R until being introduced within the interior of secondmodule 93 b. Once a predetermined amount of CWF-based fluid isintroduced into second module 93 b, valve 117B is closed. Transferliquid within second module 93 b is consequently caused to be propelledby the released CWF-based fluid through conduits 141 and 153 indirections S and M towards hydraulic turbine 96 to generate electricitywhich is able to be supplied to the grid.

The same process performed in the first discharging cycle is repeated inthe second discharging cycle, but with respect to the other module.Accordingly, the transfer liquid that has flowed into reservoir 98 isreturned by first pump 108 to first module 93 a in anticipation ofanother discharge cycle while first module liquid inlet valve 102A,first module gas outlet valve 113A and return valve 156 are opened,flowing through return conduit 158 and conduits 123 and 159 indirections N and T. In response, CWF-based gas is displaced by thedelivered transfer liquid from a seventh port of first module 93 a togas holder 92 through conduits 131, 149 and 161 in directions U, P andQ. The second discharging cycle is terminated when second module 93 b isfilled completely with CWF-based gas at a predetermined low pressure andfirst module 93 a is filled completely with transfer liquid atnear-atmospheric pressure. All valves are then closed.

The discharging cycles are likewise able to be repeated, insofar as thefirst and second modules are alternately filled with transfer liquid andCWF-based gas at a predetermined low pressure, respectively, at eachsubsequent discharging cycle. By delivering the transfer liquid back andforth between the first and second modules during each subsequentdischarging cycle and receiving CWF-based gas within the module fromwhich transfer liquid was discharged from the one or more storage tanks,the two modules are immediately ready for use in a subsequentdischarging cycle upon termination of the previous discharging cycle.The discharging process will continue until the pressure of theCWF-based fluid in the one or more storage tanks 94 a-c is reduced tothe predetermined pre-compressed pressure.

As may be appreciated by the preceding description, reduced costs andincreased system-wide round trip efficiency for mechanical energystorage relative to prior art practice are able to be realized with thesystem of the present invention by using conventional hydraulicequipment, i.e., a hydraulic pump during the charging mode and ahydraulic turbine during the discharging mode. Cost reduction is alsomade possible by avoiding the need of additional thermal energy storagethat is usually required by the prior art to absorb the heat ofcompression, since the working fluid used in the system of the presentinvention is advantageously able to undergo substantially isothermalcompression.

While some embodiments of the invention have been described by way ofillustration, it will be apparent that the invention can be carried outwith many modifications, variations and adaptations, and with the use ofnumerous equivalents or alternative solutions that are within the scopeof persons skilled in the art, without exceeding the scope of theclaims.

What is claimed is:
 1. A method for producing power with stored energy,comprising the steps of: a) providing a first pressure vessel and asecond pressure vessel, wherein said second pressure vessel is capableof being in fluid communication with said first pressure vessel and isset to a temperature no greater than ambient temperatures; b)substantially isothermally compressing, within said first pressurevessel, a condensable working fluid (CWF) that is condensable at ambienttemperatures during direct contact with an unvaporizable liquid andcausing at least a portion of the compressed CWF to be transferred fromsaid first pressure vessel to said second pressure vessel in response tointeraction with the unvaporizable liquid; c) transferring an additionalamount of compressed CWF to said second pressure vessel to cause all ora majority of the CWF within said second pressure vessel to becompressed to its saturation pressure and be condensed to produce liquidCWF; d) propelling at least some of the unvaporizable liquid located insaid first pressure vessel by the compressed CWF discharged from saidsecond pressure vessel towards at least one hydraulic turbine; and e)rotatably driving said at least one hydraulic turbine by the propelledunvaporizable liquid to produce power, wherein flow of the unvaporizableliquid to said second pressure vessel is prevented while the at least aportion of the compressed CWF is being transferred from said firstpressure vessel to said second pressure vessel.
 2. The method accordingto claim 1, wherein the CWF is substantially isothermally compressed byactivating a unit of mixing equipment in fluid communication with thefirst or second pressure vessel and thereby eliminating temperaturegradients within the CWF.
 3. The method according to claim 2, whereinthe unit of mixing equipment is activated in response to a sensedcondition indicative of liquefaction of the CWF.
 4. The method accordingto claim 1, wherein the at least some of the unvaporizable liquidlocated in said first pressure vessel is propelled by the compressed CWFdischarged from said second pressure vessel towards the at least onehydraulic turbine when the discharged compressed CWF is in a liquidstate, a gas state or in a multiphase state.
 5. The method according toclaim 1, wherein the step of causing at least a portion of thecompressed CWF to be transferred from said first pressure vessel to saidsecond pressure vessel is performed during a plurality of chargingcycles.
 6. The method according to claim 5, wherein the step ofpropelling at least some of the unvaporizable liquid located in saidfirst pressure vessel by the compressed CWF discharged from said secondpressure vessel is performed during a plurality of discharging cycles.7. The method according to claim 6, further comprising the steps of: i.providing a third pressure vessel that is capable of being in fluidcommunication with said first pressure vessel, wherein, prior toperforming one of a charging cycle or a discharging cycle, said thirdpressure vessel is completely filled with a first fluid selected fromCWF gas or the unvaporizable liquid and said first pressure vessel iscompletely filled with a second fluid selected from CWF gas or theunvaporizable liquid which is different than the first fluid; and ii.performing the one of a charging cycle or a discharging cycle such that,when terminated, said third pressure vessel is completely filled withthe second fluid and said first pressure vessel is completely filledwith the first fluid.
 8. The method according to claim 7, furthercomprising performing the other of a charging cycle or a dischargingcycle when said third pressure vessel is completely filled with thesecond fluid and said first pressure vessel is completely filled withthe first fluid.
 9. The method according to claim 1, wherein thecompressed CWF discharged from said second pressure vessel expandssubstantially isothermally during direct contact with the unvaporizableliquid within the first pressure vessel.